1. Field of the Invention
Embodiments of the invention relate to a display device and a method for driving the same.
2. Discussion of the Related Art
Various flat panel displays such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, a plasma display panel (PDP), and a field emission display (FED) haven been used.
The liquid crystal display displays an image by controlling an electric field applied to liquid crystal molecules based on a data voltage. An active matrix liquid crystal display reduces the manufacturing cost and improves a performance due to the development of process technology and driving technology. Hence, the active matrix liquid crystal display is applied to almost all of display devices from a small-sized mobile device to a large-sized television and has been widely used.
The OLED display displays an input image using a phenomenon, in which the OLED of a pixel emits light when electrons and holes are combined in an organic layer through a current flowing in a fluorescence or phosphorescence organic thin film. Because the OLED display is a self-emission display device, the OLED display may be manufactured to have lower power consumption and thinner profile than the liquid crystal display requiring a backlight unit. Further, the OLED display has advantages of a wide viewing angle and a fast response time. As a process technology of the OLED display has been developed to a large-screen mass production technology, the OLED display has expanded its market while competing with the liquid crystal display.
Each pixel of the OLED display includes an organic light emitting diode (OLED) having a self-emitting structure. An organic compound layer including a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, an electron injection layer EIL, etc. is stacked between an anode and a cathode of the OLED.
The OLED display may be variously classified depending on kinds of emission materials, an emission method, an emission structure, a driving method, and the like. The OLED display may be classified into a fluorescent emission type and a phosphorescent emission type depending on the emission method. Further, the OLED display may be classified into a top emission type and a bottom emission type depending on the emission structure. Further, the OLED display may be classified into a passive matrix OLED (PMOLED) display and an active matrix OLED (AMOLED) display depending on the driving method. It is easy to implement the OLED display as a transparent display.
Each pixel of the OLED display includes a driving thin film transistor (TFT) controlling a driving current flowing in the OLED depending on data of the input image. Device characteristics of the driving TFT, such as a threshold voltage and a mobility of the driving TFT, may change depending on a process deviation, a driving time, a driving environment, etc. The pixels of the OLED display are degraded due to changes in the device characteristics of the driving TFTs. The degradation of the pixels leads to a reduction in image quality and lifespan of the OLED display. Thus, the OLED display has adopted a technology for sensing changes in the device characteristics of the pixels and properly modifying input data based on the sensing result to compensate for the degradation of the pixels. The changes in the device characteristics of the pixels include changes in the characteristics of the driving TFT including the threshold voltage, the mobility, etc. of the driving TFT.
The pixels of the OLED display are degraded in proportion to a driving time. For example, a gate bias stress of the driving TFT is greater than a voltage applied to a gate of the driving TFT and increases in proportion to the driving time. The device characteristics of the driving TFT are changed by the gate bias stress. Because the pixels of the OLED display are driven in each frame, stresses of the pixels increase and also a stress deviation between the pixels increases. The changes in the device characteristics of the pixels lead to the degradation of the image quality of the OLED display. In case of an interior display or a public display, because the pixels have to display the same image for a long time, a stress deviation between the pixels further increases. As a result, the image quality is degraded more quickly.